Peer to Peer in CANopen - How does it work?

Question

CANopen have something called an 11 bit COB-ID, it contains function code of 4 bits + node ID of 7 bits. For example:

Image from here.

Does this mean if the MASTER send a request to the CLIENT for example send a command to SDO. Then the COB ID will be (0b1100 << 7) | node ID in binary, because 0b1100 is the function code for SDO Receive.

So then the CLIENT got that request and then the CLIENT response back to the MASTER. Should the CLIENT respons with the SDO Transmit then e.g (0b1011 << 7) | node ID ?

That sounds terrible wrong because the COB ID request did not contain any source address like SAE J1939 does.

So if I send a SDO request to an CLIENT with the node ID 0x5, then the CLIENT response back with the same node ID 0x5 or is it another node ID e.g destination ID (in this case, node ID 0x0 because that's the master node ID)?

Answer 1

There is no master, there is no client. Please don't invent your own terms which don't exist in CANopen, it's very confusing for yourself and others. There is just a sender and a receiver.

Then the COB ID will be (0b1100 << 7) | node ID in binary, because 0b1100 is the function code for SDO Receive.

Yes. So for example to configure node 1 you'd send a SDO with COBID 0x601. The node will respond with 0x581. In this case, node 1 does not care who the request comes from, only that it was addressed to node 1. Similarly, it doesn't care who picks up the response, because that is irrelevant. SDO is used to configure one single node, so only that node's id is relevant to the communication.

So the simplest and most common form of SDO communication is just a "ping pong" of 0x601 + 0x581 back and forth between the two nodes involved. Other SDO protocol versions exist, in order to utilize the bus more efficiently with less overhead, but that's another story.

Also notably you can re-configure all COBID to something else, what you see in that table is just the standard default settings. For example one node's TPDO must be another node's RPDO or they won't communicate. This pairing and configuration is exactly why CANopen is far from "plug & play", you must typically set up every node on the bus to suit the system design.